Combustion is a popular method for waste disposal. For complete combustion of gaseous waste, the fuel to air ratio should be optimized. Too little oxygen may result in waste not being completely burned during the residence time of the waste within the combustion chamber and too much oxygen may cause the flame to extinguish if the flame temperature decreases below the flash point. The oxygen demand for a given waste to be burned completely is dependent on the nature of the waste. Widely varying organic materials have vastly different heats of combustion and therefore require different amounts of oxygen for complete combustion.
This presents a serious problem for waste streams whose compositions or concentrations vary in an unpredictable manner. When materials with large heats of combustion are burned, there may not be enough oxygen available for complete combustion. When the heat of combustion of the waste is too low, insufficient heat may be produced to sustain the flame, resulting in flame extinction. Flame outs and incomplete combustion can cause releases of waste into the environment and/or plant shutdowns.
Current methods used in the control of waste combustion typically involve measuring the flame temperature and the oxygen content after burning the waste and then regulating the flow of auxiliary fuel and oxygen source in order to maintain the desired flame temperature and a fixed excess of oxygen. These current methods make adjustments to the flame feed based on measurements taken after combustion of the sample. Thus, changes in the ratio of fuel to air can only be made after the sample for which a change in the feed gas is needed has been burned. These techniques work adequately when the changes in the composition of the wastes are slow, but sudden changes may result either in the flame being extinguished or releases of incompletely burned wastes before the changes in the flame-feed can be made. Because of the large volumes of waste typically burned in an operating incinerator, even a short period of time with an incorrect amount of oxygen could cause significant damage to the environment.
It would therefore be beneficial to provide a calorimeter which could determine the heat of combustion of a small sample of a material before incinerating the bulk of the material. Once the heat of combustion for the small sample is determined then the feed rates of sample, auxiliary fuel and oxygen can be adjusted in the incinerator to ensure complete combustion of the waste.
Current methods of determining the heat of combustion for a sample typically attempt to measure the heat of combustion directly, by measuring the temperature of a flame which results as the sample is burned. The measured heat of combustion therefore depends on the temperature of the flame. However, the temperature of the flame will vary, depending on the size of the flame. This is largely due to the fact that when a large amount of fuel is burned, the flames are thicker giving rise to radiative heat transfer effects. Therefore, a conventional calorimeter in which the sample is burned in a small-scale flame would give a vastly different result than a calorimeter using a larger flame, such as those typically used in large-scale incinerators. Accordingly, conventional calorimeters can not be used with a small amount of sample to give reliable indications of the sample's heat of combustion in an incinerator and so do not reliably indicate the optimum conditions necessary to completely incinerate the waste.
Furthermore, instruments which attempt to measure the heat of the flame directly require the transfer of heat from the flame to the measuring device. Heat transfer in these devices is usually accomplished by conduction. Materials which are typically used to conduct heat are generally not resistant to corrosion. Therefore, it is not practical to use currently available instruments in hostile environments where they quickly become corroded.
One solution to some of the problems associated with attempting to determine heats of combustion using smaller flames was described by John de Ris et al. in U.S. Pat. No. 4,637,735. This reference provides a bench scale apparatus for measuring the heat-release rate of pyrolysis vapors of a material sample. The apparatus contains two sensors: a first to sense the location of the tip of the flame and a second to detect the amount of soot being released from the sample. The amount of soot detected is used to correlate the bench scale test with the large scale flame which would actually be occurring in the incinerator's burner. However, sootiness is not a precise measurement and so does not provide the consistency needed for accurate control of the combustion of waste streams. Furthermore, soot builds up over time, making maintenance of such calorimeters a real concern.